This application is a continuation-in-part of application Ser. No. 08/092,008, filed Jul. 15, 1993, now U.S. Pat. No. 5,431,559, issued on Jul. 11, 1995.
The present invention relates to burner assemblies, and particularly to oxygen-fuel burner assemblies. More particularly, the present invention relates to a burner having a fuel-delivery system and a staged oxygen-supply system.
One challenge facing the burner industry is to design an improved burner that produces lower nitrogen oxide emissions during operation than conventional burners. Typically, an industrial burner discharges a mixture of fuel and either air or oxygen. A proper ratio of fuel and air is established to produce a combustible fuel and air mixture. Once ignited, this combustible mixture burns to produce a flame that can be used to heat various products in a wide variety of industrial applications. Combustion of fuels such as natural gas, oil, liquid propane gas, low BTU gases, and pulverized coals often produce several unwanted pollutant emissions such as nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC).
Burners that combine oxygen with an atomized fuel and oxygen mixture to produce a combustible mixture are known. See, for example, U.S. Pat. No. 5,092,760 to Brown and Coppin. Burners having oxygen-enrichment systems are also known as disclosed in the IHEA Combustion Technology Manual, Fourth Edition (1988), pp. 320-21, published by The Industrial Heating Equipment Association of Arlington, Va.
Burners were developed to burn a mixture of fuel and pure oxygen in an attempt to lower the amount of NOx produced during combustion. Atmospheric combustion air contains approximately 79% nitrogen (N2) and pure oxygen contains no nitrogen. It has been observed that the higher flame temperatures brought on by burning a mixture of fuel and pure oxygen has caused the conversion of fuel-bound N2 into NOx to increase. Additionally, new technology that allows on-site generation of combustion oxygen has been developed by oxygen suppliers. This on-site generated oxygen is not pure and can contain as much as 10% nitrogen by volume. This additional nitrogen, in contact with the high-temperature oxy-fuel flame, represents an additional source of NOx emissions.
A burner assembly designed to burn fuel more completely using a lower flame temperature would lead to lower nitrogen oxide emissions. What is needed is a burner assembly that is able to burn a fuel and oxygen mixture without generating a lot of unwanted nitrogen oxide emissions. A staged oxygen burner designed to direct oxygen to various regions of a flame produced by the burner using modular components and easily manufactured precision oxygen-flow metering apparatus would lead to lower nitrogen oxide emissions and thus be a welcomed improvement over conventional burner assemblies. Ideally, an improved staged oxygen burner would be configured to accept various fuel nozzles to permit a user to burn either fuel gas or fuel oil at the option of the user.
According to the present invention, a burner assembly is provided for combining oxygen and fuel to produce a flame. The burner assembly includes a burner block formed to include a flame chamber having inlet and outlet openings, bypass means for conducting oxygen outside of the flame chamber to the outlet opening of the flame chamber, and means for discharging fuel into the flame chamber formed in the burner block.
The burner assembly also includes an oxygen-supply housing including chamber means for receiving a supply of oxygen and a base wall adjacent to the burner block. The base wall is formed to include first aperture means for discharging oxygen from the chamber means into the flame chamber and second aperture means for discharging oxygen from the chamber means into the bypass means.
In preferred embodiments, pure oxygen under pressure is admitted into the chamber means. Some of this pressurized oxygen is discharged into the inlet opening of the flame chamber through the first aperture means formed in the base wall. The rest of this pressurized oxygen is discharged from the chamber means through the second aperture means formed in the base wall to bypass the flame chamber and follow predetermined paths to the outlet opening of the flame chamber.
Illustratively, a flow-metering device is provided to control flow of oxygen discharged through the first aperture means into the inlet opening of the flame chamber. The flow-metering device is formed to include a first-stage oxygen port controlling flow of oxygen into the inlet opening of the flame chamber. The second aperture means defines a second-stage oxygen port controlling flow of oxygen to the outlet opening of the flame chamber.
By establishing a fixed ratio between the effective cross-sectional area of the first-stage oxygen port and the effective cross-sectional area of the second-stage oxygen port, it is possible to proportion and control the relative flow of oxygen to each of the inlet and outlet openings of the flame chamber. Illustratively, a first set of holes is formed in the flow-metering device to define the first-stage oxygen port and a second set of holes is formed in the base wall to define the second-stage oxygen port. Advantageously, it is possible to change the fixed ratio described above simply by varying the diameter of the holes formed in the base wall at the time that those holes are created (e.g., drilled or milled).
Some of the pressurized oxygen discharged from the oxygen-supply housing chamber means (i.e., “first-stage oxygen”) passes through the first aperture means and the first-stage oxygen port formed in the flow-metering device and then mixes with fuel provided by the discharging means in a first-stage region inside the flame chamber. This combustible fuel and oxygen mixture can be ignited to define a flame having a root portion in the flame chamber and a tip portion outside the flame chamber.
The burner block is also formed to include oxygen-discharge ports around the outlet opening of the flame chamber and oxygen-conducting means for conducting oxygen along one or more paths through the burner block and outside of the flame chamber to the oxygen-discharge ports. The rest of the pressurized oxygen discharged from the oxygen-supply housing chamber means passes through the second aperture means formed in the base wall into the oxygen-conducting means formed in the burner block. This “second-stage” oxygen passes through the oxygen-discharge ports and is ejected from the burner block into a downstream second-stage region containing a portion of the flame and lying outside the flame chamber.
In preferred embodiments, the burner block is made of a refractory material and includes an outside wall formed to include the flame chamber inlet opening and a plurality of oxygen-admission ports around the inlet opening. The burner block also includes a furnace wall configured to lie in a furnace and formed to include the flame chamber outlet opening and the plurality of oxygen-discharge ports around the outlet opening.
Illustratively, the burner block is also formed to include a plurality of oxygen-conducting passageways. These passageways are formed during casting of the burner block. Each passageway extends through the burner body to connect one of the oxygen-admission ports to one of the oxygen-discharge ports. Essentially, these passageways are arranged to bypass the flame chamber and deliver second-stage oxygen to the second-stage region downstream of the flame chamber. Illustratively, the second-stage region lies in a furnace adjacent to the burner block and the flame produced by the burner assembly heats products in the furnace.
The oxygen-supply housing is provided to hold temporarily a supply of pressurized combustion oxygen for use in the burner assembly. In use, a continuous stream of pressurized oxygen is admitted into the oxygen-supply housing using any suitable means. Some of that pressurized oxygen is distributed to the first-stage region through the first aperture means and the rest of that pressurized oxygen is distributed by the bypass means to the second-stage region using the oxygen-conducting passageways formed in the burner block.
The burner assembly in accordance with the present invention introduces combustion oxygen into two regions or combustion zones. The first-stage combustion zone is near the root of the flame inside the flame chamber and the second-stage combustion zone is in the furnace itself in a location downstream from the flame chamber and nearer to the tip of the flame. Advantageously, by withholding a portion of the combustion oxygen from the root of the flame, the fuel partially burns and the fuel-bound nitrogen is converted into reducing agents. These nitrogenous compounds are subsequently oxidized to elemental nitrogen, thereby minimizing the generation of fuel nitrogen oxides. Also, the peak flame temperature is lowered in the fuel-rich first-stage combustion zone since the generated heat dissipates rapidly. This reduction in flame temperature reduces the formation of nitrogen oxides which are temperature-dependent. In the second-stage combustion zone, additional oxygen is injected through the burner block oxygen-discharge ports to complete combustion and optimize flame shape and length.
Illustratively, the burner assembly includes several modular components that can be assembled and changed easily. An oxygen-supply housing can be connected to or disconnected from a burner block using a frame and removable fasteners. A fuel nozzle module is mounted in the oxygen-supply housing so that it can be removed easily. By replacing a gas-fuel nozzle module with an oil-fuel nozzle module, it is possible to convert the burner assembly from a gas-burning unit to an oil-burning unit.